Adds an ability to use preallocated memory instead of file

gifdec.c

@@ -13,6 +13,79 @@
 #include <unistd.h>
 #endif
 
+#ifdef USE_ALLOCATED_SPACE
+uint8_t *emu_memory;
+size_t emu_offset;
+size_t emu_size;
+
+int emu_open(const char *path, int) {
+    emu_memory = NULL;
+    emu_offset = 0;
+    emu_size = 0;
+
+    FILE *f = fopen(path, "rb");
+    fseek(f, 0, SEEK_END);
+    emu_size = ftell(f);
+    fseek(f, 0, SEEK_SET);  /* same as rewind(f); */
+
+    emu_memory = (uint8_t *)malloc(emu_size + 1);
+    if(!emu_memory) {
+        return -1;
+    }
+
+    fread(emu_memory, emu_size, 1, f);
+    fclose(f);
+
+    return 1;
+}
+
+int emu_close(int) {
+    if(emu_memory) {
+        free(emu_memory);
+    }
+    return 0;
+}
+
+int emu_read(int, void *buffer, size_t len) {
+    if(!emu_memory) {
+        return -1;
+    }
+
+    size_t read_length = ((emu_size - 1) < emu_offset + len) ? emu_size - emu_offset : len;
+    memcpy(buffer, emu_memory + emu_offset, read_length);
+    emu_offset += read_length;
+    return read_length;
+}
+
+int emu_lseek(int, size_t value, int type) {
+    size_t last_offset = emu_offset;
+
+    switch(type) {
+    case SEEK_SET:
+        emu_offset = value;
+        break;
+    case SEEK_CUR:
+        emu_offset += value;
+        break;
+    case SEEK_END:
+        emu_offset = emu_size - 1;
+        break;
+    }
+
+    if(emu_offset >= emu_size) {
+        emu_offset = last_offset;
+        return -1;
+    }
+
+    return emu_offset;
+}
+
+#define read emu_read
+#define lseek emu_lseek
+#define open emu_open
+#define close emu_close
+#endif 
+
 #define MIN(A, B) ((A) < (B) ? (A) : (B))
 #define MAX(A, B) ((A) > (B) ? (A) : (B))